FERTILIZATION 



193 



compared with vegetative individuals (they are modified similarly of 

 course), but there is no reduction in size. Thus in Cercomonas and 

 Tetramitus, the flagellate bodies of the conjugants become more or less 

 amoeboid and distinctly plastic and viscous. Other genera exhibit 

 various stages of reduction in size (Fig. 101), although in form they still 

 resemble vegetative cells. This is the case in most of the Foraminifera 



FIG. 100. Conjugation in the Flagellate, Copromonas subtilis. From Calkins, 

 "Protozoology," after Dobell. A. Vegetative form. B. Beginning of conjuga- 

 tion of two organisms; one flagellum withdrawn. C. Continued fusion. First 

 stage in nuclear "reduction." D. Second "reducing" division (heteropolar). 

 E. Conjugation completed; "reduced" nuclei fusing. F. Zygote within cyst. 



and many other Rhizopoda ; it is rare among Flagellates (Stephanosphcera, 

 Chlamydomonas) and Ciliates. Finally, we find this reduction in size 

 accompanied by a modification in form, as in other Rhizopoda whose 

 gametes become flagellated, or where they are amoeboid in forms usually 

 flagellate or motionless. 



We have then among isogamous organisms a series of forms, at one 

 extreme of which the gametes are morphologically unmodified, at the 

 other they are diminutive and structurally modified, usually in connec- 

 tion with the motor apparatus, in such a way as to render more likely 

 the accident of their meeting, likelihood of which is largely increased 



